1. Field of the Invention
This invention relates to optical systems, and more specifically, to a resonator-enhanced optical system and method for distinguishing characteristics of artifacts such as surface features or contaminants.
2. Description of the Related Art
Optical systems are often used to provide information about the position, size and characteristics of artifacts such as a feature or “particle” on a surface. Such artifacts may be contaminants such as dust motes on the surface, defects on or in the surface such as inclusions, dislocated or misshapen device elements on a semiconductor wafer or stray material, or the artifacts may actually be features used to encode data or part of the intended surface structure.
The optical characteristics of a surface artifact reveal much about the artifact's nature, shape and size. The artifact may be reflective or absorptive, transmissive (transparent) and may scatter light and/or reflect it back along an incident path, with a corresponding change in phase, amplitude and/or polarization.
At present, only limited possibilities exist for examining a surface to locate defects or other features and determine a type of defects. One instrument that is used in surface inspection systems is a scatterometer, which can provide some scattering profile information for surface artifacts (usually over a few wide angular apertures). However, a scatterometer does not reveal information about the material type of an artifact, only the artifact's scattering profile. Also, a scatterometer also does not typically provide sufficient measurement energy to measure a very small isolated artifact and is typically used more generally to detect, for example, when a surface is contaminated with dust particles. Scatterometers are also insensitive to the height of artifacts as well as the direction of the height variations, but are very sensitive to artifact shape, which severely limits the information they can provide about the material nature of an artifact.
In general, other available systems and techniques that can observe a single artifact and reveal information about the artifact's material composition are very expensive, are typically too slow for a production environment and are generally laboratory instruments with capabilities extending far beyond those needed for the production inspection environment or for data detection applications. Such techniques include Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM) and in-place spectral analysis.
While the powerful range of measurement capability provided by the laboratory instruments mentioned above is not typically needed in a production inspection environment (typically the types of surface artifacts being detected is limited to a fairly small set of possibilities), there is no present alternative for the production inspection environment that provides the ability to distinguish amongst types of artifacts, both with respect to the cost of the instrument and the speed with which a surface can be inspected. For example, it may only be necessary in a particular production environment to determine whether a surface contaminant is dust (a room artifact) or a remnant from a process step (process contamination). If a large quantity of such artifacts are present, a scatterometer will reveal their presence, but not the nature of the artifact material.
The above-incorporated parent U.S. patent application discloses a highly sensitive resonator-enhanced measuring system having a reference resonator, which can be used to provide a relatively low-cost and very high resolution measurement system that can locate variations in surface characteristics such as height and artifact size. The reference resonator system, as well the other systems described in the U.S. patent applications incorporated in the parent application, are suitable for production environments but do not in themselves provide the ability to distinguish the material characteristics of an artifact detected on a surface of interest or examine the relation between the size of an artifact and the artifact's material composition.
Therefore, it would be desirable to provide further refinements in the optical systems described in the above-incorporated parent application and in other optical systems that can distinguish characteristics of an artifact on or in a surface of interest. It would further be desirable to provide such a system that is capable of both locating artifacts and determining their characteristics simultaneously and quickly in a production scanning or data extraction environment.